In the semiconductor industry, fuse elements are widely used in integrated circuits for a variety of purposes, such as improving manufacturing yield or customizing a generic integrated circuit. For example, by replacing defective circuits on a chip with duplicate or redundant circuits on the same chip, manufacturing yields can be significantly increased. A fuse disconnected by a laser beam is referred to as a laser fuse, while a fuse disconnected by passing an electrical current, or blowing, is referred to as an electrical fuse, or e-fuse. By selectively blowing fuses within an integrated circuit, which has multiple potential uses, a generic integrated circuit design may be economically manufactured and adapted to a variety of customer uses.
Typically, fuses are incorporated in the design of the integrated circuit, wherein the fuses are selectively blown, for example, by passing an electrical current of a sufficient magnitude to cause electromigration or melting, thereby creating a more resistive path or an open circuit. Alternatively, a current that is weaker than the current required to entirely blow the fuse can be applied in order to degrade the fuse, thus increasing a resistance through the fuse. The process of selectively blowing or degrading fuses is often referred to as “programming”.
Laser fuses are widely used. However, they suffer scalability problems. A certain size is desired for accurate blowing, thus the laser fuses cannot be scaled proportionately with other devices. For the reason, electrical fuses are preferred for small-scale integrated circuits.
A commonly used electrical fuse is shown in FIG. 1. The fuse element is a polysilicon line 2 connected to metal lines 6 through vias 4. Polysilicon line 2 is doped to lower resistivity. A programming current causes heating in polysilicon line 2, and thus an open circuit is formed. This structure may suffer reliability problems, since the resistivity of polysilicon line 2 is determined by the doping concentration, and may vary from process to process. The program voltage and program time vary accordingly.
FIG. 2A illustrates a perspective view of another electrical fuse, which includes a polysilicon plate 12 connected to metal lines 16 and 18 through tungsten contact plugs 14a and 14b, respectively. Tungsten plug 14a has a relatively small cross-sectional area compared to contact plug 14b, and is used as a fuse element. When a program current passes from one metal line to another metal line, tungsten plug 14a is blown by a high current density. This embodiment suffers scalability problems. To ensure that via 14a is blown while via 14b remains intact, via 14b has to have a significantly greater cross-sectional area, for example, about five times greater, than that of via 14a. The entire fuse structure thus occupies a relatively great chip area.
FIG. 2B illustrates a variation of the fuse structure shown in FIG. 2A, wherein tungsten plugs 14a and 14b are replaced by a rectangular-shaped tungsten contact 19, which acts as a fuse element. A polysilicon plate 12 under the tungsten contact 19 is primarily used for landing the contact 19 and not for carrying fusing current.
A possible problem for the structure is that after the contact 19 is blown by heat and an open circuit forms, remaining heat may cause silicidation of tungsten with polysilicon plate 12, and a low resistivity path of silicide reconnects the disconnected parts. The reconnection may be in the form of shorting or degrading (with a relatively high resistive, but not completely open, path).
Therefore, there is the need for a highly reliable, scalable electrical fuse, particularly for integrated circuits fabricated using 90 nm technology and below.